I own two bushmasters but I would like to build an AR15 from scratch. What basic tools do I need to start with and what are some basic do's and dont's?Is it best to purchase the lower with the trigger assembly already in it or get the parts kit seperate. Same question applies to the upper assembly as well.

So I assume that you're going to assemble your first rifle rather than actually build one from a 80% lower right?
I've always bought parts kits rather than assembled lowers.
[b]Do's:[/b]
1. Wear safty glasses.
2. Get a couple of extra front take down pin detents and springs (see #1 above).
3. Mask off the lower with thick tape (I use duct tape) while taping in the bolt stop retaining pin.
4. Use a torque wrench to properly torque the barrel to the upper (30#) three times.
5. Use the proper grease on the barrel nut (see #4).
6. Make sure you've got the right tiny springs in the right tiny holes. [:)]
7. The disconnector spring has an up and and down and works only one way.
8. Get the hammer spring on right side up.
9. Test your safety and trigger before use.
10. Clean the barrel before use.
11. Use the proper tools for a proper job.
12. Clearly think though the intended purpose for the weapon - don't try to make it do everything half assed.
[b]Don'ts[/b]
1. Don't hurry.
2. Don't skimp on the quality saving $10 on a $600 rifle that you're going to bet your life on possibly one day.
3. Worry if you nick or scratch the thing or if the colors don't match - it's a weapon.

Celebrating the Second Amendment
One Fine Firearm at a Time, Under God

Thats how I did mine.Its great putting all the springs and pins together and ending up with a finished product.Take your time and all will go well, its not a race.And its cheaper to assemble your lower.

I printed out the assembly directions and read them over until I was sick of reading them. When it came time to put everything together, it was natural and simple. I still had to refer to the directions a few times, but everything made sense.
If you ever assembled "snap together" models as a kid, you won't have any problems. It is really that simple, its just a more expensive basic model kit!